This animation shows the shapes of some elliptical orbits. These orbits have different eccentricities. If an ellipse has a high eccentricity, is it round like a circle or long like an oval?
Click on image for full size Original animation by Windows to the Universe staff (Randy Russell).

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Eccentricity of an Orbit

You may think that most objects in space that orbit something else move in
circles, but that isn't the case. Although some objects follow circular orbits,
most orbits are shaped more like "stretched out" circles or ovals.
Mathematicians and astronomers call this oval shape an ellipse.

An ellipse can be very long and thin, or it can be quite round - almost like
a circle. Scientists use a special term, "eccentricity",
to describe how round or how "stretched out" an ellipse is. If the
eccentricity of an ellipse is close to one (like 0.8 or 0.9), the ellipse is
long and skinny. If the eccentricity is close to zero, the ellipse is more
like a circle.

The eccentricity of Earth's orbit is
very small, so Earth's orbit is nearly circular. Earth's orbital eccentricity
is less than 0.02. The orbit of Pluto is
the most eccentric of any planet in our Solar
System. Pluto's orbital eccentricity is
almost
0.25.
Many comets have extremely eccentric orbits. Halley's
Comet, for instance, has an orbital eccentricity of almost 0.97!

The Sun is not at the center of an elliptical orbit. It is a little off to
one side, at a point called a "focus" of the ellipse. Because of this offset
the planet moves closer to
and
further
away
from the Sun every orbit. The close point in each orbit is called perihelion.
The far away point is called aphelion.
If an orbit has a large eccentricity, the difference between the perihelion
distance and the aphelion distance will also be large. Earth is
only 3% further from the Sun at aphelion than it is at perihelion. Pluto's
aphelion distance from the Sun is 66% greater than its perihelion distance.

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